1. Field of the Invention
The present invention relates to a wired board such as a base for an IC (integrated circuit) package for hermetically sealing an electronic part such as an IC chip, and more particularly to a wired ceramic board of the type for bonding thereto an electronic part by the wire bonding technique. The present invention further relates to a method of producing a wired ceramic board of the above described type.
2. Description of the Related Art
A wired ceramic board (hereinafter will be referred to as wired board) constituting an IC package base or the like includes a plurality of bonding pads for bonding thereto an electronic part and a plurality of external connection terminals for connection to an external circuit board such as a motherboard or printed circuit board. The bonding pads and the external connection terminals include metallized layers which are formed from a high melting point metal paste which is printed onto ceramic green sheets and sintered together with the laminated green sheets. Such metallized layers constituting the bonding pads and the external connection terminals are usually coated with a non-electrolytic Ni-plated layer and further with a non-electrolytic Au-plated layer for the purpose of preventing their oxidation. The non-electrolytic plating technique is used for the reason that the metallized layers in this kind of wired board are electrically independent or separated. This kind of wired board has a die-attach surface to which an electronic part is attached by the wire bonding technique. More specifically, by such a wire bonding technique, a number of connecting terminals (i.e., electrodes) of the electronic part and a number of bonding pads (i.e., electrodes) of the wired board are bonded to each other by means of wires (i.e., metallic thin wires made of gold or aluminum and sometimes called bonding wires). One method of connecting such a wired board having mounted thereon an electronic part to a motherboard or the like circuit board is a so-called PGA (Pin Grid Array) joint. In the PGA joint, a number of connecting terminals arranged in a grid-pattern on the rear surface of the wired board are connected with pins, respectively. The pins are inserted into sockets provided to the motherboard to be received thereby. Another is a so-called LGA (Land Grid Array) joint. In the LGA joint, a number of connecting terminals (hereinafter will be referred to as LGA pads) on the rear surface of the wired board are mechanically pressed against the respective connecting terminals of the motherboard by way of fixing devices such as clips.
A disadvantage of the PGA or LGA type wired board is that it requires sockets or the like fixing means and therefore a complicated joint structure, thus increasing the cost of its connection to the motherboard. Another disadvantage is that it is large in size and requires a relatively large wire length for its connection, so it is disadvantageous from the point of view of inductance.
In place of such PGA or LGA joint, it has been proposed to utilize a BGA (Ball Grid Array) solder joint for soldering the wired board to the motherboard even if the wired board is of the type for connecting thereto an electronic part by the wire bonding technique.
In this type of wired board, the bonding pads for connection to an electronic part by the wire bonding technique needs to have a relatively thick Au-plated layer for attaining a desired bonding or solder joint between the pads and the wires. However, if the bonding pads are Au-plated so as to have a sufficiently thick Au-plated layer together with the external connection terminals for connection to an external circuit board, there arises a problem in that a thick Au-plated layer on the external connection terminals is causative of a considerable amount of intermetallic compound at the time of soldering of the external connection terminals to the external circuit board such as a motherboard, thus lowering the strength of the solder joint and therefore the reliability of same. On the contrary, if the external connection terminals are Au-plated so as to have properly thin Au-plated layers together with the bonding pads, there arises a problem in that the bonding pads with such thin Au-plated layers cannot attain desired wire bonding.
It is accordingly an object of the present invention to provide a novel and improved wired board of the type adapted for connection to an electronic part by wire bonding and for connection to an external circuit board such as a motherboard by soldering (BGA bonding), which can attain a highly reliable electrical connection by both of the wire bonding and the BGA bonding.
It is a further object of the present invention to provide a method of producing a wired board of the foregoing character.
To achieve the foregoing objects, the present invention provides a wired board which comprises a substrate, a plurality of bonding pads formed on the substrate for connection to connecting terminals of an electronic part, a plurality of external connection terminals formed on the substrate for connection to connecting terminals of an external circuit board. Each of the bonding pads has a metallized layer formed on the substrate, and further has a Nixe2x80x94B-plated layer, a Nixe2x80x94P-plated layer and a Au-plated layer which are formed in this order on the metallized layer. The Au-plated layer has the thickness ranging from 1.2 to 3.5 xcexcm. Each of the external connection terminals has a metallized layer formed on the substrate, and further has a Nixe2x80x94B-plated layer and a Au-plated layer which are formed in this order on the metallized layer of each of the external connection terminals. The Au-plated layer of each of the external connection terminals has the thickness ranging from 0.01 to 1 xcexcm.
The present invention further provides a method of producing a wired board having a substrate, a plurality of bonding pads formed on the substrate for connection to connecting terminals of an electronic part, and a plurality of external connection terminals formed on the substrate for connection to connecting terminals of an external circuit board, each of the bonding pads having a metallized layer formed on the substrate and each of the external connection terminals having a metallized layer formed on the substrate. The method comprises the steps of Nixe2x80x94B-plating the metallized layer of each of the bonding pads and the metallized layer of each of the external connection terminals so as to form Nixe2x80x94B-plated layers thereon, respectively, Au-plating the Nixe2x80x94B-plated layers so as to form thereon Au-plated layers of the thickness ranging from 0.01 to 1 xcexcm, masking, after the step of the Au-plating, the external connection terminals so as to protect the external connection terminals from plating, removing the Au-plated layer formed on the Nixe2x80x94B-plated layer of each of the bonding pads, Nixe2x80x94P-plating the Nixe2x80x94B-plated layer of each of the bonding pads so as to form thereon a Nixe2x80x94P-plated layer, and Au-plating the Nixe2x80x94P-plated layer of each of the bonding pads so as to form thereon a Au-plated layer having the thickness ranging from 1.2 to 3.5 xcexcm.